This invention relates to the electrostatic measurement art, and more particularly to a new and improved method and apparatus for determining the charge or voltage distribution of an electrophotographic surface.
One area of use of the present invention is in determining the charge or voltage distribution and ultimately the surface quality and uniformity of an electrophotographic surface such as the surface of a photoconductive drum used in photocopiers, although the principles of the present invention can be variously applied to inspecting any surface capable of holding electrical charge. In the past, photoconducting drums have been inspected visually or by light scattering techniques to determine the presence of surface defects such as holes. Recently, there has been provided a method and apparatus for quickly and reliably determining the physical quality and uniformity of a charged surface in a manner providing a measure of the charge properties of the surface. Such method and apparatus is disclosed in U.S. Pat. No. 5,101,159 issued Mar. 31, 1992, the disclosure of which is hereby incorporated by reference, and the method and apparatus has the capability of determining the size, number and location of surface defects. In such method and apparatus, important considerations are minimizing electrical noise and maximizing resolution of detected signals. These considerations, in turn, lead to the requirements of selecting the area of the sensor to be sufficiently small so as to minimize electrical noise when the sensor is in close proximity to the surface, and of maintaining a constant distance between the sensor and the surface during relative movement therebetween so that the current in the sensor is proportional only to the change in voltage along the surface and the speed at which the surface change variation crosses the edge of the sensor.
In the approach taught in U.S. Pat. No. 5,101,159, the requirement of making the sensor of sufficiently small area and the requirement to maintain a constant distance between the sensor and drum surface requires the use of expensive techniques for manufacturing the sensors while requiring the use of expensive precision mechanical machine components which require frequent maintenance to hold constant the sensor to drum distance. For example, the mechanical system which holds the drum and produces drum motion is subject to wear which produces subsequent eccentric drum rotation and mechanical noise which will produce motion in the distance between the sensor and drum while mechanically generated vibrations produced on the sensor board will produce relative motion between the sensor and drum.
Due to the introduction of charge on the drum surface during testing, a voltage in the order of 500 to 1000 volts is produced on the drum surface relative to the ground referenced potential of the sensor electrode(s). This requires that a minimum distance be established and maintained therebetween to prevent arc-over and subsequent damage to the sensor or drum. This minimum distance, as establish by the well known Paschen rule for arc-over spacings, dictates a spacing which is greater than that which would be desired to produce high signal strength for any value of non-uniformity of the charge on the surface to be measured, and which would produce high spatial resolution capability of the measurement. The actual minimum spacing which can be allowed by the prior method and apparatus referenced above also has to be based on the allowance for unstable distance variation due to machine operation and mechanical wear, in view of the Paschen constraint.
It would, therefore, be highly desirable to provide such a method and apparatus which eliminates the requirements of selecting a sensor area to be sufficiently small and holding the spacing between sensor and drum constant to reduce noise, while obtaining the advantages of increased signal strength and increased measurement resolution.